The present invention relates to a layer system for electrochemical probes.
More particularly, it relates to a layer system with a ceramic substrate in which electrodes are applied and which carries a sensor layer applied over the electrodes and also means for improving adherence of the sensor layer to the substrate.
Layer systems of the above-mentioned general type are known in the art. Such layer systems are utilized, for example, for producing resistance measuring sensors for determination of oxygen content in exhaust gases. In such a system a semi-conductor metal oxide layer with a temperature-dependent or gas concentration-dependent resistance is applied on a ceramic substrate. In addition, the metal oxide layer is covered with a protective layer. The disadvantage of this layer system is that the adherence between the substrate and the metal oxide is relatively poor, since the substrate has a flat and smooth surface. In particular, a probe used in a motor vehicle for determination of the oxygen content in exhaust gases is subjected to extremely high thermal loads. The different thermal expansion coefficients of the substrate and the metal oxide layer lead to the separation of the metal oxide layer from the substrate.
European document EP-A 140 340 discloses a substrate with dispersed ceramic particles which by sintering provide an intimate connection with the substrate. A rough surface is produced by the particles which are firmly connected with the substrate. This method requires that the ceramic particles cover the whole surface and therefore adhere both to the substrate and to the electrode surface. As a result, the contact surface between the sensor layer and the electrodes is reduced. Moreover, the application of the ceramic particles requires a different technology than the thick layer and screen printing technology used for producing the probes.